[GHI #60] Use real token and fix default icon vertical alignment

[Jeremy-Walton]

Task

#60

What Changed

• Use correct token for icon line height
• Use middle for icon vertical alignment so it lines up correctly when used with text.
• Add documentation for icons

Sanity Check

• Have you updated any usage of changed tokens?
• Have you updated the docs with any component changes?
• Do you need to update the package version?

Screenshots